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Full text of "Handbook Of Chemical Engineering - I"

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The two systems of vacuum filtering that may be practiced, are termed "wet" and "dry." In the wet system the liquid drawn through the filtering medium passes through the vacuum apparatus. In the dry means are employed between the fJter and vacuum apparatus for removing the liquid. The wet system does not permit of maintaining a high vacuum since there must be sufficient clearance of the moving parts of the vacuum apparatus to handle liquid. If there be grit in the filtrate the moving parts of the vacuum apparatus in contact with the filtrate will be attacked. Rotary vacuum pumps are commonly employed in the wet system and it has the advantage of simplicity and can be employed to advantage on solids which filter readily and with clear and non-corrosive filtrates. A vacuum pump made of acid-resisting materials adds further cost and complication.
The various pieces of accessory apparatus can of course be placed where they are most convenient and where a battery of filters is connected into the accessory apparatus "the only point to be observed beyond securing the necessary capacities throughout the accessory apparatus is to see that the piping has sufficient fall to secure drainage. The vacuum receivers in the dry system are fitted with a float and vacuum release so that if anything goes wrong with the solution pumps the vacuum apparatus will not be flooded.
It is usual to allow % cu. ft. of displacement in the vacuum apparatus to every square foot of filtering area. To obtain the displacement of a compressor subtract one-half the net cross-section of the piston rod from the area of the cylinder, convert the latter into square feet and multiply by twice the stroke expressed in feet or fractions of a foot and the number of revolutions per minute. The "free air " capacity of compressors at sea level as given by the manufacturers is the displacement and can be used as a practical factor in determining the proper size compressor to use for vacuum purposes.
From 15 to 25 in. vacuum is maintained depending upon altitude. The higher the altitude the less the vacuum which can be maintained. The appended table shows the relation between atmospheric pressure in pounds and mercury reading in inches. Inches divided by 2.04 will give pounds.
Altitude above sea level, feet	Atmospheric pressure, pounds per square inch	Barometer reading, inches of mercury	Altitude above sea level, feet	Atmospheric pressure, pounds per square inch	Barometer reading, inches of mercury
0	14.7	30.0	5,500	11.9	24.3
500	14.4	29.4	6,000	11.7	23.8
1,000	14.2	28.8	6,500	11.5	23.4
1,500	13.9	28.3	7,000	11.2	22.9
2,000	13.6	27.7	7,500	11.0	22.5
2;500	13.4	27.2	8,000	10.8	22.1
3,000	13.1	26.7	8,500	10.6	21.7
3,500	12.9	26.2	9,000	10.4	21.2
4,000	12.6	25.7	9,500	10.2	20.8
4,500	12.4	25.2	10,000	10.0	20.4
5.000	12.1	24.7			
Power Required.—In compression work the pressure on one side of the piston is the atmospheric or very close to the atmospheric and on the other the outlet